- Email: [email protected]
Preparation and characterisation of electrophoretically deposited hydroxyapatite coatings on type 316L stainless steel
Corrosion Science 45 (2003) 237–252 www.elsevier.com/locate/corsci
Preparation and characterisation of electrophoretically deposited hydroxyapatite coatings on type 316L stainless steel T.M. Sridhar a, U. Kamachi Mudali
b,*
, M. Subbaiyan
a
a
b
Department of Analytical Chemistry, University of Madras, Chennai 600 025, India Corrosion Science and Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India Received 4 January 2001; accepted 22 April 2002
Abstract Hydroxyapatite (HAP) coatings were developed on type 316L stainless steel (SS) by electrophoretic deposition at various deposition potentials from 30 to 90 V using the stoichiometric HAP (Ca/P ratio 1.67) powder in a suspension of isopropyl alcohol. The optimum coating parameters were established at 60 V and 3 min, after vacuum sintering at 800 °C. The phase purity of the coated surface was confirmed by XRD and secondary ion mass spectrometry confirmed the presence of both Ca and P on the coated layers. The electrochemical corrosion parameters Ecorr (open circuit potential) and pitting potentials, evaluated in Hank’s solution shifted towards noble direction for the HAP coated specimens in comparison with uncoated type 316L SS. Electrochemical impedance spectroscopic investigations revealed higher polarisation resistance and lower capacitance values after immersing the coated specimens in Hanks solution for 200 h. This indicates the stable nature of the coatings formed. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Hydroxyapatite coatings; Polarisation; Impedance; Corrosion
*
Corresponding author. Tel.: +91-4114-480202; fax: +91-4114-480301/480060. E-mail address: [email protected] (U. Kamachi Mudali).
0010-938X/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 2 ) 0 0 0 9 1 - 4
238
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
1. Introduction Metals and alloys are used in restoration of anatomical structures for centuries owing to their superior mechanical properties. However, the degradation of most metals implanted in the human body has narrowed the choice of clinically usable metals and alloys to mainly––stainless steels, cobalt–chromium and titanium and its alloys [1,2]. These metallic devices are unique that they are exposed to living cells, tissues and biological fluids which are not only dynamic but are also a hostile environment for the survival of the implant [3]. Type 316L stainless steel (SS) are widely used for implantation purposes in orthopaedic surgery owing to their corrosion resistance, mechanical properties and low cost. However, clinical experience has shown that they are susceptible to localised corrosion in the human body causing the release of metal ions into the tissues surrounding the implants. Several incidences of failures [4,5] of such devices have demanded the application of biocompatible and corrosion resistant coatings and surface modification of the alloys. There is widespread interest in the basic calcium phosphate mineral, hydroxyapatite (Ca10 (PO4 )6 (OH)2 ) (HAP) which is the prototype of one of the major constituent of bones and teeth. It is biocompatible and bioactive and is capable of interacting with the surrounding bone. HAP is known to have a simulating effect on bone formation, which is known as osseo-induction. It enhances the osseo-ıntergration, and there are indications that chemical bonding may occur between HAP and bone [6]. But, its poor mechanical properties inhibit its use for implantation purposes. Hence, it is proposed in this work to develop thin layers of HAP on the surface of type 316L SS by electrophoretic deposition and to study their electrochemical properties for applications as orthopaedic devices. The excellent biocompatibility and biostability of HAP coatings have become well established and the use of this material for prosthetic applications are being rapidly popularised in the past few years [7]. The dominant requirements connected with the development of HAP coatings on metallic implants are––preparation of stoichiometric powder material with required chemical and phase composition. This is established by their chemical identity (Ca/P ratio 1.66) and by close crystallographical affinity with bone tissue. Another essential criteria are their deposition as coatings without the presence of non-stoichiometric phases of the powder. A number of novel methods [8] offering the potential for better control of film structure for coating HAP include hot isostatic pressing, flame spraying, ion beam deposition, laser ablation and electrochemical deposition along with plasma spraying which has been widely studied over the decade. The major problems associated with plasma spraying process are that it is a line of sight process that produces a non-uniform coating with heterogeneous structure. The high temperature involved alters the HAP and metal substrate phases [9]. Hence, electrophoretic deposition of HAP on metal substrates was used to overcome the above drawbacks and to achieve the uniform distribution of fine HAP deposits. The advantages of this technique include high purity of layers formed, ease of obtaining the desired thickness and stronger adhesion to the substrate. The present work was undertaken to optimise the applied coating potential required to produce adherent
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
239
HAP coatings on SS and electrochemically evaluate their corrosion resistance and impedance behaviour in simulated body conditions.
2. Experimental work 2.1. Materials HAP powder was chemically synthesised by wet chemical method using H3 PO4 (0.3 M) and Ca(OH)2 (0.1 M) solutions at the optimum conditions developed in our laboratory. The reaction temperature was maintained at 60 °C and pH of the solution at 10. After the addition was complete, the samples are subjected to ripening treatment which included refluxing for 30 min followed by stirring for 1 h and the solution was aged for 24 h. The precipitate was then filtered and sintered at 900 °C for an hour in an air furnace. The synthesised powder was characterised by using IR, laser Raman, thermal and XRD studies to confirm its chemical nature (Ca/P ratio 1.66) and crystallographic structure. Type 316L SS electrodes of 10 10 2 mm size samples were mechanically polished from 120 to 1000 grit SiC papers followed by using a 1 lm diamond paste to get a mirror finish. The electrodes were washed thoroughly with running distilled water, rinsed and ultrasonically degreased with acetone and dried. The electrophoretic deposition process is carried out at room temperature from a 2.5% suspension of HAP in isopropyl alcohol in a 100 ml glass beaker, closed with a rubber cork with Teflon covering. The suspension was stirred using a Teflon paddle at 1000 rpm with a magnetic stirrer. A 316L SS sheet of 30 90 1 mm dimension was used as the anode and the working electrode was used as the cathode. The distance between the two electrodes was 1 cm. Deposition was carried out on a 1 cm2 surface area on one side of the specimen. The other side and edges were masked with a non-conducting Teflon tape. The weight gain and the thickness of the coatings are measured and calculated, and only coatings with uniform characteristics are selected for further studies. A minimum of five samples was coated for each varying coating parameter. HAP was electrophoretically deposited on type 316L SS samples of 1 cm2 area from a 2.5% suspension in isopropanol. The electrophoretic yield on 316L SS metal substrate was determined at various applied voltages between 30 and 90 V at a constant time of 3 min. The sintering of the coatings after electrophoretic deposition was carried out at 800 °C for an hour in a vacuum furnace (105 Torr) [10]. X-ray diffraction patterns were recorded with a SIMENS D-500 diffractometer to study the phase changes and depth profiling of the coating with a CAMECA make secondary ion mass spectrometry (SIMS) instrument with IMS-4f ion microscope–ion microprobe. 2.2. Potentiodynamic polarisation and electrochemical impedance studies Potentiodynamic cyclic polarisation of the coated samples was carried out under simulated body fluid––Hank’s solution (NaCl: 8.0, CaCl2 : 0.14, KCl: 0.40, NaHCO3 :
240
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
0.35, Glucose: 1.00, NaH2 PO4 : 0.10, MgCl2 6H2 O: 0.10, Na2 HPO4 2H2 O: 0.06, MgSO4 7H2 O: 0.06 g/l respectively) at pH 7.4 and temperature 37 1 °C. A saturated calomel electrode (SCE) and a platinised platinum black were used as the reference and auxiliary electrodes respectively. The solution was deaerated with pure argon gas throughout the experiment. The area of the coated surface exposed to corrosion study was 1 cm2 . The other sides of the electrode and its edges were masked with lacquer and were dried in air. The electrodes were further dipped into the electrolytic solution to study the corrosion process. Similar procedure was followed for uncoated type 316L SS also. The electrochemical measurements carried out on the coated samples are open circuit potential (OCP)––time measurements, cyclic polarisation and electrochemical impedance spectroscopic studies. The critical parameters like corrosion potential (Ecorr ), the breakdown potential (Eb ) and repassivation potential (Ep ), were evaluated from the polarisation curves. The samples were immersed in Hank’s solution and the OCP (Ecorr ) was monitored for an hour. During cyclic polarisation study, the potential was increased from 0.200 V below the OCP towards the noble direction at a rate of 10 mV/min until the breakdown potential (Eb ) was attained where the alloy enters the pitting region. The sweep direction was then reversed after reaching an anodic current density of 1.0 mA/cm2 until the reverse scan reached the passive region. The electrochemical experiments were performed using Solartron make Electrochemical Interface Model––SI 1287 and Frequency Response Analyser Model––SI 1255. Electrochemical impedance tests were carried out at OCP condition before and after polarisation, using a frequency scan from 20 kHz to 0.1 Hz. The impedance measurements were carried out before and after polarisation in order to evaluate the performance of the coating in equilibrium condition and after the onset of the corrosion process respectively. EIS was also carried out to study the changes in the coating behaviour after ageing the samples in the test solution at OCP condition over a period of 200 h. Five sets of the uncoated and coated samples obtained under the optimum coating parameters were immersed in separate glass cells.
3. Results and discussion 3.1. Open circuit potential–time measurements The OCP–time plots for the uncoated and HAP coated type 316L SS are shown in Fig. 1. The OCP of uncoated sample shifted towards active direction and reached a potential of 0.237 V (vs SCE) after 60 min. This could be due to the dissolution that could occur at the alloy surface [11]. The OCP–time curves of the coated HAP specimens shifted towards noble direction. This indicates the protective nature of the coatings on 316L SS. The noble behaviour of the ceramic HAP coatings could be due to the insulating nature of the surface. However, the HAP coatings obtained are microporous in nature and hence the electrolyte seeps into the coating through the pores. Thus the OCP–time behaviour of HAP coatings developed at different potentials showed variations in the noble behaviour. The coatings obtained at a po-
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
241
Fig. 1. OCP–time measurements in Hank’s solution of uncoated and HAP coated type 316L SS obtained with varying coating potentials at a constant time of 3 min.
tential of 60 V and 3 min performed better. It showed an initial OCP of þ0.039 V (vs SCE), reached a steady state within 10 min and attained a potential of þ0.095 V (vs SCE) after 60 min. The curves obtained for the coatings with deposition potential more than 70 V showed minor variations in OCP. This may be due to the existence of a large diffusion path within such thick coatings, which are capable of causing large potential drops. From the OCP–time measurements it can be inferred that at the coating voltage of 60 V, the OCP was shifted to a maximum in the nobler region and the time taken to attain constant OCP was faster and more significant when compared with samples uncoated. 3.2. Cyclic polarisation studies The potentiodynamic anodic cyclic polarisation curves for HAP coated samples along with the uncoated 316L SS are shown in Fig. 2a and b respectively. The values of Ecorr , Eb and Ep for uncoated 316L SS were found to be at 0.237, þ0.352 and 0.179 V (vs SCE) respectively. The hysteresis loop with considerable area was observed for 316L SS, which indicates the extensive operation of pit propagation mechanism [12]. The breakdown potential (Eb ) and repassivation potentials (Ep ) for the samples coated at 60 V for 3 min was found to be at þ0.567 V and þ0.160 V (vs
242
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
Fig. 2. (a) and (b) Cyclic polarisation curves in Hank’s solution of uncoated and HAP coated type 316L SS obtained with varying coating potentials at a constant time of 3 min with a scan rate of 10 mV/min.
SCE) respectively. However, the breakdown potential (Eb ) for 30, 50, 70 and 90 V remains at þ0.471, þ0.524, þ0.501 and þ0.449 V (vs SCE) respectively, which indicates the optimal coating potential range to vary from 50 to 70 V. The area of hysteresis loop for the coated samples is also smaller when compared with the uncoated type 316L SS. The breakdown potentials for all the coated samples were
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
243
found to be nobler than uncoated 316L SS, thus exhibiting improved corrosion resistance. The decrease in the value of breakdown potentials (Eb ) with the increase in coating voltage can be attributed to the thickness and porosity of coating and subsequent weakening of substrate coating bond strength. The coating weight increases with increases in applied voltage and hence its coating thickness. The coatings obtained at 20 V resulted in incoherent coatings while above 70 V and 5 min they were thicker and highly porous. Physical disintegration of thick coatings was observed for samples coated above 90 V, on immersion in the electrolyte. The likely mechanism is a deagglomeration of the particles accompanied by subsequent interparticle fissure formation and eventually particle detachment from the substrate [13]. During electrophoretic deposition, hydrogen evolution increases with increases with increase in coating voltage and time, which results in an increase in porosity and pore size of the coating. The presence of large pores increases the surface area exposed to the electrolytic attack. This is well in agreement with the reports that as the pore size increases the breakdown potential decrease and hence its corrosion resistance decreases [14,15]. The above results suggest that 30–70 V is the effective coating range for obtaining HAP coatings and the optimal applied potential for electrophoretic deposition of HAP on type 316L SS is 60 V and 3 min. It is evident from the results that HAP coated 316L SS exhibit enhanced Eb and Ep values, suggesting an improvement in the pitting corrosion resistance in Hank’s solution.
3.3. Electrochemical impedance spectroscopic studies The Nyquist and Bode plots obtained for the uncoated type 316L SS and HAP coated metal substrates at different coating potentials before and after polarisation conditions are given in Figs. 3–6a and b respectively. The total impedance jZj, polarisation resistance (Rp ), and the capacitance ðCÞ values obtained are given in Table 1. Impedance results showed that the polarisation resistance of uncoated 316L SS and HAP coated samples decreased after polarisation. Similarly, there was a decrease in the total impedance jZj, for the uncoated samples on comparison with the uncoated samples indicating the better performance of HAP coated samples. An increase in capacitance and a decrease in polarisation resistance values from 4:393 105 to 12:32 105 F/cm2 and 3:170 105 to 1:053 105 X cm2 respectively were obtained for uncoated 316L SS. For the coated sample at the optimum coating parameters a decrease in capacitance and a marginal decrease in polarisation resistance was observed. This indicates that the coatings offer better resistance to corrosion. The impedance results further indicate that the corrosion process once initiated is further inhibited by the presence of coating. Thus, the presence of HAP coating acts as a stable barrier in increasing the corrosion resistance. The marginal decrease in jZj and polarization resistance of the coatings after polarisation could be associated with electrolyte penetration into the coating and the subsequent electrochemical reactions at the HAP–metal interface [16].
244
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
Fig. 3. Nyquist (a) and Bode (b) plots in Hank’s solution for uncoated and HAP coated type 316L SS for various coating potentials at a constant time of 3 min before polarisation.
Lower values of Rp and higher C, obtained after polarisation from the impedance plots for uncoated 316L SS. The impedance parameters of the coated samples indicate an increase in corrosion resistance due to the presence of HAP coatings. A marginal decrease in the values is observed for the coated samples after the onset of corrosion. The HAP coatings thus tend to protect the alloy from further attack and penetration of anions from the electrolyte. This could be due to the ex-
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
245
Fig. 4. Nyquist (a) and Bode (b) plots in Hank’s solution for uncoated and HAP coated type 316L SS for various coating potentials at a constant time of 3 min before polarisation.
istence of a HAP–metal interface obtained on sintering of the coatings. Its presence is further confirmed by SIMS investigations. The other contributing factor is the microporosity of the coatings. After polarisation, the corrosion products formed during the corrosion process could block the pores. This is revealed from the
246
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
Fig. 5. Nyquist (a) and Bode (b) plots in Hank’s solution for uncoated and HAP coated type 316L SS for various coating potentials at a constant time of 3 min after polarisation.
impedance values obtained. This also indicates that the coating now acts as a barrier which could decrease the rate of attack of anions. Hence the porous nature of the coating acts as an effective insulator resulting in a decrease in capacitance values. From the Bode plots no major changes in the phase angle were observed for all the samples before and after polarisation.
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
247
Fig. 6. Nyquist (a) and Bode (b) plots in Hank’s solution for uncoated and HAP coated type 316L SS for various coating potentials at a constant time of 3 min after polarisation.
3.4. Ageing studies The changes in the impedance behaviour of the HAP coatings after immersing the samples in the test solution at OCP condition for 200 h were recorded. The values of impedance parameters were found to be higher for the coated samples and are given in Table 2. The values decreased for the uncoated type 316L SS samples
248
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
Table 1 Impedance parameters of uncoated and HAP coated type 316L SS in Hank’s solution Electrophoretic coating potential
Uncoated 30 50 60 70 90
Before polarisation
After polarisation
Total impedance, jZj ( 104 X cm2 )
Polarisation resistance, Rp ( 105 X cm2 )
Capacitance, C ( 105 F/cm2 )
Total impedance, jZj ( 104 X cm2 )
Polarisation resistance, Rp ( 105 X cm2 )
Capacitance, C ( 105 F/cm2 )
5.963 8.566 11.351 18.58 9.463 6.835
3.170 5.998 13.96 101.17 9.60 7.443
4.393 4.012 3.982 2.910 4.002 4.113
2.176 6.213 8.225 15.23 5.568 3.967
1.053 3.958 10.85 59.21 5.411 2.253
12.32 3.152 2.541 0.912 3.716 3.910
Table 2 Impedance parameters of uncoated type 316L SS and HAP coated samples after ageing in Hank’s solution No. of hours
1 10 50 100 150 200
Uncoated 316L SS
HAP coated 316L SS
Total impedance, jZj ( 104 X cm2 )
Polarisation resistance, Rp ( 105 X cm2 )
Capacitance, C ( 105 F/cm2 )
Total impedance, jZj ( 104 X cm2 )
Polarisation resistance, Rp ( 105 X cm2 )
Capacitance, C ( 105 F/cm2 )
5.963 3.863 3.297 2.863 2.341 1.998
3.170 4.866 3.927 3.197 2.650 2.343
4.393 4.487 5.055 5.753 6.093 6.441
18.58 19.33 21.97 22.29 23.01 23.87
101.17 105.36 106.44 108.91 109.11 109.57
2.910 2.755 2.137 1.942 1.716 1.310
indicating the susceptibility of the bare alloy towards corrosion on immersion in Hank’s solution. The increased susceptibility towards corrosion attack of the uncoated surface could be due to the formation of active pitting sites on the alloy surface as a result of thinning of the passive film. Whereas the trends for the coated samples indicate that the coatings prevent the surface from attack of anion with no major dissolution of the coating. This could be due to the blocking of the micropores by the presence of Ca2þ , PO4 3 and other ions present in the solution [17]. This can result in the reduction of pathways by which the HAP coatings impedes the ingress of anions in the electrolyte from attacking the metal–HAP interface. It was further confirmed by SIMS investigations. After ageing in Hank’s solution, the bond strength of HAP–metal could have increased significantly as no flaking off of the coatings and increase in microfissures was observed [18]. These trends for the coated samples indicate that the coatings are stable on immersion and prevent the surface from attack of anion with no major dissolution of the coating.
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
249
Fig. 7. XRD patterns of HAP coated type 316L SS obtained at the optimum coating parameters of 60 V and 3 min.
3.5. XRD studies Fig. 7 shows the XRD pattern of the HAP coated type 316L SS obtained by electrophoretic deposition at a constant potential of 60 V and 3 min after vacuum sintering. The presence of diffraction peaks with minimal line broadening and high intensities, which indicate a well crystallised material. The strongest lines in this XRD pattern corresponds to reflections at 0 0 2, 2 1 1, 1 1 2, 3 0 0, 2 0 2, 3 1 0, 2 2 2 and 2 1 3 planes of HAP after indexing with the JCPDS file no.9-432 [19]. The patterns showed no structural transformation either in crystallinity or stoichiometry and confirm the stable nature of the HAP coatings formed after thermal treatment with no extraneous peaks, thus indicating the presence of stoichiometric HAP with a Ca/P ratio of 1.67. 3.6. Secondary ion mass spectrometry The change in the composition of the coating with the depth of the coating was recorded using SIMS and the profile obtained is given in Fig. 8. The intensities of Ca2þ is of the highest order as it forms the major constituent of HAP, followed by phosphorous and oxygen. The intensity of the Ca2þ signal gradually decreases on sputtering, but the concentration does not fall to lower levels even on sputtering above 3000 s i.e., after sputtering into the alloy region. This indicates that Ca2þ has diffused into the passive layer of 316L SS and bonds with the alloying elements,
250
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
Fig. 8. SIMS depth profile of HAP coated type 316L SS.
which is responsible for the adherence of the coatings. The intensity of the alloying elements Fe and Cr are very low at the start of sputtering and slowly begin to increase as the HAP layer is removed. This indicates that Ca2þ present in HAP has diffused into the passive layer of the metal, leading to the formation of a strong metal–ceramic interface, which improves the adhesion of the coating and acts as a barrier in impeding the penetration of chloride ions that initiates pitting attack.
4. Discussion The coating thickness increases with the increase in applied potential during EPD. At lower coating potentials (<30 V) thinner coatings are obtained whereas at higher potentials (>90 V) the coatings are highly porous and fragile in nature. The coatings obtained in the potential range from 30 to 90 V were uniform and stable after vacuum sintering at 800 °C for one hour and on immersion in the electrolyte. No flaking, cracks or decohesion of the coatings was observed on sintering and after the electrochemical studies. This is evident from the noble values obtained from corrosion experiments. Further, no metal or metal oxide peaks were detected on the surface of the HAP coating by XRD and the SIMS results clearly establish the fact that the coatings are intact. The breakdown potential increases with increase in coating potential up to 60 V and then tends to decrease. Similarly the jZj and Rp tend to decrease with increase in coating potential to 60 V and decreases thereafter for both conditions of impedance study i.e before and after polarization. But, the capacitance decreases up to 60 V and
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
251
increases thereafter. The lower capacitance values recorded after the onset of corrosion process indicate that the coatings effectively resist the attack of chloride ions from the electrolyte. The lower C values on ageing confirm that the coatings act as diffusion barriers thus decrease in the Cl ion attack in solution. These results indicate that 60 V and 3 min to be the optimum coating parameters for producing HAP coatings on type 316L SS. In case of the uncoated type 316L SS the OCP values moves towards the active direction accompanied by lower breakdown potential, jZj and Rp along with higher capacitance values in comparison with the coated samples. The values of jZj, Rp and C decreases with the onset of corrosion process i.e after polarization conditions and also on immersion for 200 h indicating a decrease in corrosion resistance and a less protective passivating film as pit growth occurs. This clearly indicates the susceptibility of the metal to attack of chloride ions as the pitting has initiated after polarisation, which could further result in a decrease in pH within the pits. This enhances the loss of metal ions as metal chlorides resulting in a decrease in the frequency. The electrochemical studies of HAP coated samples exhibit their enhanced corrosion resistance in simulated body fluid––Hank’s environment. The higher and stable values of jZj and Rp and very low values of C are obtained on immersion for 200 h. This indicates that the surfaces are more corrosion resistant probably due to the formation of a strongly diffused HAP coating on type 316L SS and is found to be optimum at 60 V and 3 min. The presence of phase pure stoichiometric HAP is confirmed by XRD studies thus ensuring the biostability of HAP phases, since the presence of non-stoichiometric phases of HAP would result in early dissolution of the coatings in vivo. Further the low intensity of alloying elements obtained from SIMS data indicate the diffusion of HAP into the metal substrate, accounting for the improved corrosion resistance of HAP coated type 316L SS. Thus, the results of the above studies indicate that HAP coatings can be deployed as a viable alternative for improving the corrosion resistance of type 316L SS for enhancing the biocompatibility of the implant devices.
5. Conclusions The optimum coating parameters for electrophoretic deposition of HAP on type 316L SS was established at 60 V and 3 min. XRD and SIMS studies confirm the presence of stoichiomertic structure and presence of Ca and P on depth profiling of the HAP coatings. The OCP and breakdown potentials of HAP coated samples shifted towards the nobler direction when compared with the uncoated 316L SS. Marginal changes observed in the impedance parameters (jZj, Rp and C) for the coated samples before and after polarisation indicating the stable nature of the coatings. The increase in polarisation resistance and a decrease in capacitance on immersion for 200 h indicate that HAP coatings are electrochemically stable and evince improved corrosion resistance in simulated body fluids.
252
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
Acknowledgements The authors are thankful to Dr. H.S. Khatak, Head, Corrosion Science and Technology Division (CSTD) and Dr. R.K. Dayal, Head, ACSSS, CSTD, IGCAR, Kalpakkam, for their constant support and encouragement during the course of the present investigation. Thanks are due to Dr. S. Rajagopalan, MSD for help in SIMS experiments and Mr. V.S. Sastry and Mr. K.L.N. Reddy, MSD for X-ray studies. One of the author’s (T.M.S) acknowledge the Council of Scientific and Industrial Research, New Delhi, for providing financial assistance during this work.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
D.F. Williams, J. Mater. Sci. 22 (1987) 3421. N.D. Greene, D.A. Jones, J. Mater. 1 (1966) 345. M. Sivakumar, U. Kamachi Mudali, S. Rajeswari, J. Mater. Eng. Perf. 3 (1994) 744. M. Sivakumar, U. Kamachi Mudali, S. Rajeswari, J. Mater. Sci. Lett. 13 (1994) 142. O.E.M. Pholer, Failure of orthopaedic metallic implants, in: ASM Handbook on Failure Analysis and Prevention, 9th ed., vol. 11, ASM International, Metals Park, Ohio, 1986, p. 670. M. Jarcho, Clin. Ortho. Rel. Res. 157 (1981) 259. P. Ducheyne, L.L. Hench, A. Kagan, M. Martens, A. Burssens, J.C. Mulier, J. Biomed. Mater. Res. 14 (1980) 225. T.M. Sridhar, T.K. Arumugam, S. Rajeswari, M. Subbaiyan, J. Mater. Sci. Lett. 16 (1997) 1964. S.R. Radin, P. Ducheyne, J. Mater. Sci. Mater. Med. 3 (1992) 33. T.M. Sridhar, U. Kamachi Mudali, M. Subbaiyan, Corros. Sci. (submitted). J. Gluszek, J. Masalski, Brit. Corros. J. 27 (1992) 135. J.W. Oldfield, Int. Mate. Rev. 32 (1987) 153. P. Ducheyne, S. Radin, M. Heughebaert, J.C. Heughebaert, Biomaterials 11 (1990) 244. K.H.W. Seah, R. Thampuran, S.H. Teoh, Corros. Sci. 40 (1998) 547. T. Raghu, S.N. Malhotra, P. Ramakrishnan, Corrosion 45 (1989) 698. J.F. McIntyre, J.F. Leidheiser, Ind. Eng. Chem. Prod. Res. Dev. (1985). F. Deflorian, L. Fedrizzi, S. Rossi, P.L. Bonora, Electrochim. Acta 44 (1999) 4243. H.G. Pfaff, G. Willmann, R. Pothig, Properties of HA-coatings, in: P. Ducheyne, D. Christiansen (Eds.), Bioceramics, vol. 6, Butterworth-Heinemann, London, 1993, pp. 421–424. K. de Groot, J.G.C. Wolke, J.A. Jansen, Proc. Instr. Mech. Eng. 212 (1988) 137.
Preparation and characterisation of electrophoretically deposited hydroxyapatite coatings on type 316L stainless steel T.M. Sridhar a, U. Kamachi Mudali
b,*
, M. Subbaiyan
a
a
b
Department of Analytical Chemistry, University of Madras, Chennai 600 025, India Corrosion Science and Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India Received 4 January 2001; accepted 22 April 2002
Abstract Hydroxyapatite (HAP) coatings were developed on type 316L stainless steel (SS) by electrophoretic deposition at various deposition potentials from 30 to 90 V using the stoichiometric HAP (Ca/P ratio 1.67) powder in a suspension of isopropyl alcohol. The optimum coating parameters were established at 60 V and 3 min, after vacuum sintering at 800 °C. The phase purity of the coated surface was confirmed by XRD and secondary ion mass spectrometry confirmed the presence of both Ca and P on the coated layers. The electrochemical corrosion parameters Ecorr (open circuit potential) and pitting potentials, evaluated in Hank’s solution shifted towards noble direction for the HAP coated specimens in comparison with uncoated type 316L SS. Electrochemical impedance spectroscopic investigations revealed higher polarisation resistance and lower capacitance values after immersing the coated specimens in Hanks solution for 200 h. This indicates the stable nature of the coatings formed. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Hydroxyapatite coatings; Polarisation; Impedance; Corrosion
*
Corresponding author. Tel.: +91-4114-480202; fax: +91-4114-480301/480060. E-mail address: [email protected] (U. Kamachi Mudali).
0010-938X/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 2 ) 0 0 0 9 1 - 4
238
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
1. Introduction Metals and alloys are used in restoration of anatomical structures for centuries owing to their superior mechanical properties. However, the degradation of most metals implanted in the human body has narrowed the choice of clinically usable metals and alloys to mainly––stainless steels, cobalt–chromium and titanium and its alloys [1,2]. These metallic devices are unique that they are exposed to living cells, tissues and biological fluids which are not only dynamic but are also a hostile environment for the survival of the implant [3]. Type 316L stainless steel (SS) are widely used for implantation purposes in orthopaedic surgery owing to their corrosion resistance, mechanical properties and low cost. However, clinical experience has shown that they are susceptible to localised corrosion in the human body causing the release of metal ions into the tissues surrounding the implants. Several incidences of failures [4,5] of such devices have demanded the application of biocompatible and corrosion resistant coatings and surface modification of the alloys. There is widespread interest in the basic calcium phosphate mineral, hydroxyapatite (Ca10 (PO4 )6 (OH)2 ) (HAP) which is the prototype of one of the major constituent of bones and teeth. It is biocompatible and bioactive and is capable of interacting with the surrounding bone. HAP is known to have a simulating effect on bone formation, which is known as osseo-induction. It enhances the osseo-ıntergration, and there are indications that chemical bonding may occur between HAP and bone [6]. But, its poor mechanical properties inhibit its use for implantation purposes. Hence, it is proposed in this work to develop thin layers of HAP on the surface of type 316L SS by electrophoretic deposition and to study their electrochemical properties for applications as orthopaedic devices. The excellent biocompatibility and biostability of HAP coatings have become well established and the use of this material for prosthetic applications are being rapidly popularised in the past few years [7]. The dominant requirements connected with the development of HAP coatings on metallic implants are––preparation of stoichiometric powder material with required chemical and phase composition. This is established by their chemical identity (Ca/P ratio 1.66) and by close crystallographical affinity with bone tissue. Another essential criteria are their deposition as coatings without the presence of non-stoichiometric phases of the powder. A number of novel methods [8] offering the potential for better control of film structure for coating HAP include hot isostatic pressing, flame spraying, ion beam deposition, laser ablation and electrochemical deposition along with plasma spraying which has been widely studied over the decade. The major problems associated with plasma spraying process are that it is a line of sight process that produces a non-uniform coating with heterogeneous structure. The high temperature involved alters the HAP and metal substrate phases [9]. Hence, electrophoretic deposition of HAP on metal substrates was used to overcome the above drawbacks and to achieve the uniform distribution of fine HAP deposits. The advantages of this technique include high purity of layers formed, ease of obtaining the desired thickness and stronger adhesion to the substrate. The present work was undertaken to optimise the applied coating potential required to produce adherent
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
239
HAP coatings on SS and electrochemically evaluate their corrosion resistance and impedance behaviour in simulated body conditions.
2. Experimental work 2.1. Materials HAP powder was chemically synthesised by wet chemical method using H3 PO4 (0.3 M) and Ca(OH)2 (0.1 M) solutions at the optimum conditions developed in our laboratory. The reaction temperature was maintained at 60 °C and pH of the solution at 10. After the addition was complete, the samples are subjected to ripening treatment which included refluxing for 30 min followed by stirring for 1 h and the solution was aged for 24 h. The precipitate was then filtered and sintered at 900 °C for an hour in an air furnace. The synthesised powder was characterised by using IR, laser Raman, thermal and XRD studies to confirm its chemical nature (Ca/P ratio 1.66) and crystallographic structure. Type 316L SS electrodes of 10 10 2 mm size samples were mechanically polished from 120 to 1000 grit SiC papers followed by using a 1 lm diamond paste to get a mirror finish. The electrodes were washed thoroughly with running distilled water, rinsed and ultrasonically degreased with acetone and dried. The electrophoretic deposition process is carried out at room temperature from a 2.5% suspension of HAP in isopropyl alcohol in a 100 ml glass beaker, closed with a rubber cork with Teflon covering. The suspension was stirred using a Teflon paddle at 1000 rpm with a magnetic stirrer. A 316L SS sheet of 30 90 1 mm dimension was used as the anode and the working electrode was used as the cathode. The distance between the two electrodes was 1 cm. Deposition was carried out on a 1 cm2 surface area on one side of the specimen. The other side and edges were masked with a non-conducting Teflon tape. The weight gain and the thickness of the coatings are measured and calculated, and only coatings with uniform characteristics are selected for further studies. A minimum of five samples was coated for each varying coating parameter. HAP was electrophoretically deposited on type 316L SS samples of 1 cm2 area from a 2.5% suspension in isopropanol. The electrophoretic yield on 316L SS metal substrate was determined at various applied voltages between 30 and 90 V at a constant time of 3 min. The sintering of the coatings after electrophoretic deposition was carried out at 800 °C for an hour in a vacuum furnace (105 Torr) [10]. X-ray diffraction patterns were recorded with a SIMENS D-500 diffractometer to study the phase changes and depth profiling of the coating with a CAMECA make secondary ion mass spectrometry (SIMS) instrument with IMS-4f ion microscope–ion microprobe. 2.2. Potentiodynamic polarisation and electrochemical impedance studies Potentiodynamic cyclic polarisation of the coated samples was carried out under simulated body fluid––Hank’s solution (NaCl: 8.0, CaCl2 : 0.14, KCl: 0.40, NaHCO3 :
240
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
0.35, Glucose: 1.00, NaH2 PO4 : 0.10, MgCl2 6H2 O: 0.10, Na2 HPO4 2H2 O: 0.06, MgSO4 7H2 O: 0.06 g/l respectively) at pH 7.4 and temperature 37 1 °C. A saturated calomel electrode (SCE) and a platinised platinum black were used as the reference and auxiliary electrodes respectively. The solution was deaerated with pure argon gas throughout the experiment. The area of the coated surface exposed to corrosion study was 1 cm2 . The other sides of the electrode and its edges were masked with lacquer and were dried in air. The electrodes were further dipped into the electrolytic solution to study the corrosion process. Similar procedure was followed for uncoated type 316L SS also. The electrochemical measurements carried out on the coated samples are open circuit potential (OCP)––time measurements, cyclic polarisation and electrochemical impedance spectroscopic studies. The critical parameters like corrosion potential (Ecorr ), the breakdown potential (Eb ) and repassivation potential (Ep ), were evaluated from the polarisation curves. The samples were immersed in Hank’s solution and the OCP (Ecorr ) was monitored for an hour. During cyclic polarisation study, the potential was increased from 0.200 V below the OCP towards the noble direction at a rate of 10 mV/min until the breakdown potential (Eb ) was attained where the alloy enters the pitting region. The sweep direction was then reversed after reaching an anodic current density of 1.0 mA/cm2 until the reverse scan reached the passive region. The electrochemical experiments were performed using Solartron make Electrochemical Interface Model––SI 1287 and Frequency Response Analyser Model––SI 1255. Electrochemical impedance tests were carried out at OCP condition before and after polarisation, using a frequency scan from 20 kHz to 0.1 Hz. The impedance measurements were carried out before and after polarisation in order to evaluate the performance of the coating in equilibrium condition and after the onset of the corrosion process respectively. EIS was also carried out to study the changes in the coating behaviour after ageing the samples in the test solution at OCP condition over a period of 200 h. Five sets of the uncoated and coated samples obtained under the optimum coating parameters were immersed in separate glass cells.
3. Results and discussion 3.1. Open circuit potential–time measurements The OCP–time plots for the uncoated and HAP coated type 316L SS are shown in Fig. 1. The OCP of uncoated sample shifted towards active direction and reached a potential of 0.237 V (vs SCE) after 60 min. This could be due to the dissolution that could occur at the alloy surface [11]. The OCP–time curves of the coated HAP specimens shifted towards noble direction. This indicates the protective nature of the coatings on 316L SS. The noble behaviour of the ceramic HAP coatings could be due to the insulating nature of the surface. However, the HAP coatings obtained are microporous in nature and hence the electrolyte seeps into the coating through the pores. Thus the OCP–time behaviour of HAP coatings developed at different potentials showed variations in the noble behaviour. The coatings obtained at a po-
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
241
Fig. 1. OCP–time measurements in Hank’s solution of uncoated and HAP coated type 316L SS obtained with varying coating potentials at a constant time of 3 min.
tential of 60 V and 3 min performed better. It showed an initial OCP of þ0.039 V (vs SCE), reached a steady state within 10 min and attained a potential of þ0.095 V (vs SCE) after 60 min. The curves obtained for the coatings with deposition potential more than 70 V showed minor variations in OCP. This may be due to the existence of a large diffusion path within such thick coatings, which are capable of causing large potential drops. From the OCP–time measurements it can be inferred that at the coating voltage of 60 V, the OCP was shifted to a maximum in the nobler region and the time taken to attain constant OCP was faster and more significant when compared with samples uncoated. 3.2. Cyclic polarisation studies The potentiodynamic anodic cyclic polarisation curves for HAP coated samples along with the uncoated 316L SS are shown in Fig. 2a and b respectively. The values of Ecorr , Eb and Ep for uncoated 316L SS were found to be at 0.237, þ0.352 and 0.179 V (vs SCE) respectively. The hysteresis loop with considerable area was observed for 316L SS, which indicates the extensive operation of pit propagation mechanism [12]. The breakdown potential (Eb ) and repassivation potentials (Ep ) for the samples coated at 60 V for 3 min was found to be at þ0.567 V and þ0.160 V (vs
242
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
Fig. 2. (a) and (b) Cyclic polarisation curves in Hank’s solution of uncoated and HAP coated type 316L SS obtained with varying coating potentials at a constant time of 3 min with a scan rate of 10 mV/min.
SCE) respectively. However, the breakdown potential (Eb ) for 30, 50, 70 and 90 V remains at þ0.471, þ0.524, þ0.501 and þ0.449 V (vs SCE) respectively, which indicates the optimal coating potential range to vary from 50 to 70 V. The area of hysteresis loop for the coated samples is also smaller when compared with the uncoated type 316L SS. The breakdown potentials for all the coated samples were
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
243
found to be nobler than uncoated 316L SS, thus exhibiting improved corrosion resistance. The decrease in the value of breakdown potentials (Eb ) with the increase in coating voltage can be attributed to the thickness and porosity of coating and subsequent weakening of substrate coating bond strength. The coating weight increases with increases in applied voltage and hence its coating thickness. The coatings obtained at 20 V resulted in incoherent coatings while above 70 V and 5 min they were thicker and highly porous. Physical disintegration of thick coatings was observed for samples coated above 90 V, on immersion in the electrolyte. The likely mechanism is a deagglomeration of the particles accompanied by subsequent interparticle fissure formation and eventually particle detachment from the substrate [13]. During electrophoretic deposition, hydrogen evolution increases with increases with increase in coating voltage and time, which results in an increase in porosity and pore size of the coating. The presence of large pores increases the surface area exposed to the electrolytic attack. This is well in agreement with the reports that as the pore size increases the breakdown potential decrease and hence its corrosion resistance decreases [14,15]. The above results suggest that 30–70 V is the effective coating range for obtaining HAP coatings and the optimal applied potential for electrophoretic deposition of HAP on type 316L SS is 60 V and 3 min. It is evident from the results that HAP coated 316L SS exhibit enhanced Eb and Ep values, suggesting an improvement in the pitting corrosion resistance in Hank’s solution.
3.3. Electrochemical impedance spectroscopic studies The Nyquist and Bode plots obtained for the uncoated type 316L SS and HAP coated metal substrates at different coating potentials before and after polarisation conditions are given in Figs. 3–6a and b respectively. The total impedance jZj, polarisation resistance (Rp ), and the capacitance ðCÞ values obtained are given in Table 1. Impedance results showed that the polarisation resistance of uncoated 316L SS and HAP coated samples decreased after polarisation. Similarly, there was a decrease in the total impedance jZj, for the uncoated samples on comparison with the uncoated samples indicating the better performance of HAP coated samples. An increase in capacitance and a decrease in polarisation resistance values from 4:393 105 to 12:32 105 F/cm2 and 3:170 105 to 1:053 105 X cm2 respectively were obtained for uncoated 316L SS. For the coated sample at the optimum coating parameters a decrease in capacitance and a marginal decrease in polarisation resistance was observed. This indicates that the coatings offer better resistance to corrosion. The impedance results further indicate that the corrosion process once initiated is further inhibited by the presence of coating. Thus, the presence of HAP coating acts as a stable barrier in increasing the corrosion resistance. The marginal decrease in jZj and polarization resistance of the coatings after polarisation could be associated with electrolyte penetration into the coating and the subsequent electrochemical reactions at the HAP–metal interface [16].
244
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
Fig. 3. Nyquist (a) and Bode (b) plots in Hank’s solution for uncoated and HAP coated type 316L SS for various coating potentials at a constant time of 3 min before polarisation.
Lower values of Rp and higher C, obtained after polarisation from the impedance plots for uncoated 316L SS. The impedance parameters of the coated samples indicate an increase in corrosion resistance due to the presence of HAP coatings. A marginal decrease in the values is observed for the coated samples after the onset of corrosion. The HAP coatings thus tend to protect the alloy from further attack and penetration of anions from the electrolyte. This could be due to the ex-
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
245
Fig. 4. Nyquist (a) and Bode (b) plots in Hank’s solution for uncoated and HAP coated type 316L SS for various coating potentials at a constant time of 3 min before polarisation.
istence of a HAP–metal interface obtained on sintering of the coatings. Its presence is further confirmed by SIMS investigations. The other contributing factor is the microporosity of the coatings. After polarisation, the corrosion products formed during the corrosion process could block the pores. This is revealed from the
246
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
Fig. 5. Nyquist (a) and Bode (b) plots in Hank’s solution for uncoated and HAP coated type 316L SS for various coating potentials at a constant time of 3 min after polarisation.
impedance values obtained. This also indicates that the coating now acts as a barrier which could decrease the rate of attack of anions. Hence the porous nature of the coating acts as an effective insulator resulting in a decrease in capacitance values. From the Bode plots no major changes in the phase angle were observed for all the samples before and after polarisation.
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
247
Fig. 6. Nyquist (a) and Bode (b) plots in Hank’s solution for uncoated and HAP coated type 316L SS for various coating potentials at a constant time of 3 min after polarisation.
3.4. Ageing studies The changes in the impedance behaviour of the HAP coatings after immersing the samples in the test solution at OCP condition for 200 h were recorded. The values of impedance parameters were found to be higher for the coated samples and are given in Table 2. The values decreased for the uncoated type 316L SS samples
248
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
Table 1 Impedance parameters of uncoated and HAP coated type 316L SS in Hank’s solution Electrophoretic coating potential
Uncoated 30 50 60 70 90
Before polarisation
After polarisation
Total impedance, jZj ( 104 X cm2 )
Polarisation resistance, Rp ( 105 X cm2 )
Capacitance, C ( 105 F/cm2 )
Total impedance, jZj ( 104 X cm2 )
Polarisation resistance, Rp ( 105 X cm2 )
Capacitance, C ( 105 F/cm2 )
5.963 8.566 11.351 18.58 9.463 6.835
3.170 5.998 13.96 101.17 9.60 7.443
4.393 4.012 3.982 2.910 4.002 4.113
2.176 6.213 8.225 15.23 5.568 3.967
1.053 3.958 10.85 59.21 5.411 2.253
12.32 3.152 2.541 0.912 3.716 3.910
Table 2 Impedance parameters of uncoated type 316L SS and HAP coated samples after ageing in Hank’s solution No. of hours
1 10 50 100 150 200
Uncoated 316L SS
HAP coated 316L SS
Total impedance, jZj ( 104 X cm2 )
Polarisation resistance, Rp ( 105 X cm2 )
Capacitance, C ( 105 F/cm2 )
Total impedance, jZj ( 104 X cm2 )
Polarisation resistance, Rp ( 105 X cm2 )
Capacitance, C ( 105 F/cm2 )
5.963 3.863 3.297 2.863 2.341 1.998
3.170 4.866 3.927 3.197 2.650 2.343
4.393 4.487 5.055 5.753 6.093 6.441
18.58 19.33 21.97 22.29 23.01 23.87
101.17 105.36 106.44 108.91 109.11 109.57
2.910 2.755 2.137 1.942 1.716 1.310
indicating the susceptibility of the bare alloy towards corrosion on immersion in Hank’s solution. The increased susceptibility towards corrosion attack of the uncoated surface could be due to the formation of active pitting sites on the alloy surface as a result of thinning of the passive film. Whereas the trends for the coated samples indicate that the coatings prevent the surface from attack of anion with no major dissolution of the coating. This could be due to the blocking of the micropores by the presence of Ca2þ , PO4 3 and other ions present in the solution [17]. This can result in the reduction of pathways by which the HAP coatings impedes the ingress of anions in the electrolyte from attacking the metal–HAP interface. It was further confirmed by SIMS investigations. After ageing in Hank’s solution, the bond strength of HAP–metal could have increased significantly as no flaking off of the coatings and increase in microfissures was observed [18]. These trends for the coated samples indicate that the coatings are stable on immersion and prevent the surface from attack of anion with no major dissolution of the coating.
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
249
Fig. 7. XRD patterns of HAP coated type 316L SS obtained at the optimum coating parameters of 60 V and 3 min.
3.5. XRD studies Fig. 7 shows the XRD pattern of the HAP coated type 316L SS obtained by electrophoretic deposition at a constant potential of 60 V and 3 min after vacuum sintering. The presence of diffraction peaks with minimal line broadening and high intensities, which indicate a well crystallised material. The strongest lines in this XRD pattern corresponds to reflections at 0 0 2, 2 1 1, 1 1 2, 3 0 0, 2 0 2, 3 1 0, 2 2 2 and 2 1 3 planes of HAP after indexing with the JCPDS file no.9-432 [19]. The patterns showed no structural transformation either in crystallinity or stoichiometry and confirm the stable nature of the HAP coatings formed after thermal treatment with no extraneous peaks, thus indicating the presence of stoichiometric HAP with a Ca/P ratio of 1.67. 3.6. Secondary ion mass spectrometry The change in the composition of the coating with the depth of the coating was recorded using SIMS and the profile obtained is given in Fig. 8. The intensities of Ca2þ is of the highest order as it forms the major constituent of HAP, followed by phosphorous and oxygen. The intensity of the Ca2þ signal gradually decreases on sputtering, but the concentration does not fall to lower levels even on sputtering above 3000 s i.e., after sputtering into the alloy region. This indicates that Ca2þ has diffused into the passive layer of 316L SS and bonds with the alloying elements,
250
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
Fig. 8. SIMS depth profile of HAP coated type 316L SS.
which is responsible for the adherence of the coatings. The intensity of the alloying elements Fe and Cr are very low at the start of sputtering and slowly begin to increase as the HAP layer is removed. This indicates that Ca2þ present in HAP has diffused into the passive layer of the metal, leading to the formation of a strong metal–ceramic interface, which improves the adhesion of the coating and acts as a barrier in impeding the penetration of chloride ions that initiates pitting attack.
4. Discussion The coating thickness increases with the increase in applied potential during EPD. At lower coating potentials (<30 V) thinner coatings are obtained whereas at higher potentials (>90 V) the coatings are highly porous and fragile in nature. The coatings obtained in the potential range from 30 to 90 V were uniform and stable after vacuum sintering at 800 °C for one hour and on immersion in the electrolyte. No flaking, cracks or decohesion of the coatings was observed on sintering and after the electrochemical studies. This is evident from the noble values obtained from corrosion experiments. Further, no metal or metal oxide peaks were detected on the surface of the HAP coating by XRD and the SIMS results clearly establish the fact that the coatings are intact. The breakdown potential increases with increase in coating potential up to 60 V and then tends to decrease. Similarly the jZj and Rp tend to decrease with increase in coating potential to 60 V and decreases thereafter for both conditions of impedance study i.e before and after polarization. But, the capacitance decreases up to 60 V and
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
251
increases thereafter. The lower capacitance values recorded after the onset of corrosion process indicate that the coatings effectively resist the attack of chloride ions from the electrolyte. The lower C values on ageing confirm that the coatings act as diffusion barriers thus decrease in the Cl ion attack in solution. These results indicate that 60 V and 3 min to be the optimum coating parameters for producing HAP coatings on type 316L SS. In case of the uncoated type 316L SS the OCP values moves towards the active direction accompanied by lower breakdown potential, jZj and Rp along with higher capacitance values in comparison with the coated samples. The values of jZj, Rp and C decreases with the onset of corrosion process i.e after polarization conditions and also on immersion for 200 h indicating a decrease in corrosion resistance and a less protective passivating film as pit growth occurs. This clearly indicates the susceptibility of the metal to attack of chloride ions as the pitting has initiated after polarisation, which could further result in a decrease in pH within the pits. This enhances the loss of metal ions as metal chlorides resulting in a decrease in the frequency. The electrochemical studies of HAP coated samples exhibit their enhanced corrosion resistance in simulated body fluid––Hank’s environment. The higher and stable values of jZj and Rp and very low values of C are obtained on immersion for 200 h. This indicates that the surfaces are more corrosion resistant probably due to the formation of a strongly diffused HAP coating on type 316L SS and is found to be optimum at 60 V and 3 min. The presence of phase pure stoichiometric HAP is confirmed by XRD studies thus ensuring the biostability of HAP phases, since the presence of non-stoichiometric phases of HAP would result in early dissolution of the coatings in vivo. Further the low intensity of alloying elements obtained from SIMS data indicate the diffusion of HAP into the metal substrate, accounting for the improved corrosion resistance of HAP coated type 316L SS. Thus, the results of the above studies indicate that HAP coatings can be deployed as a viable alternative for improving the corrosion resistance of type 316L SS for enhancing the biocompatibility of the implant devices.
5. Conclusions The optimum coating parameters for electrophoretic deposition of HAP on type 316L SS was established at 60 V and 3 min. XRD and SIMS studies confirm the presence of stoichiomertic structure and presence of Ca and P on depth profiling of the HAP coatings. The OCP and breakdown potentials of HAP coated samples shifted towards the nobler direction when compared with the uncoated 316L SS. Marginal changes observed in the impedance parameters (jZj, Rp and C) for the coated samples before and after polarisation indicating the stable nature of the coatings. The increase in polarisation resistance and a decrease in capacitance on immersion for 200 h indicate that HAP coatings are electrochemically stable and evince improved corrosion resistance in simulated body fluids.
252
T.M. Sridhar et al. / Corrosion Science 45 (2003) 237–252
Acknowledgements The authors are thankful to Dr. H.S. Khatak, Head, Corrosion Science and Technology Division (CSTD) and Dr. R.K. Dayal, Head, ACSSS, CSTD, IGCAR, Kalpakkam, for their constant support and encouragement during the course of the present investigation. Thanks are due to Dr. S. Rajagopalan, MSD for help in SIMS experiments and Mr. V.S. Sastry and Mr. K.L.N. Reddy, MSD for X-ray studies. One of the author’s (T.M.S) acknowledge the Council of Scientific and Industrial Research, New Delhi, for providing financial assistance during this work.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
D.F. Williams, J. Mater. Sci. 22 (1987) 3421. N.D. Greene, D.A. Jones, J. Mater. 1 (1966) 345. M. Sivakumar, U. Kamachi Mudali, S. Rajeswari, J. Mater. Eng. Perf. 3 (1994) 744. M. Sivakumar, U. Kamachi Mudali, S. Rajeswari, J. Mater. Sci. Lett. 13 (1994) 142. O.E.M. Pholer, Failure of orthopaedic metallic implants, in: ASM Handbook on Failure Analysis and Prevention, 9th ed., vol. 11, ASM International, Metals Park, Ohio, 1986, p. 670. M. Jarcho, Clin. Ortho. Rel. Res. 157 (1981) 259. P. Ducheyne, L.L. Hench, A. Kagan, M. Martens, A. Burssens, J.C. Mulier, J. Biomed. Mater. Res. 14 (1980) 225. T.M. Sridhar, T.K. Arumugam, S. Rajeswari, M. Subbaiyan, J. Mater. Sci. Lett. 16 (1997) 1964. S.R. Radin, P. Ducheyne, J. Mater. Sci. Mater. Med. 3 (1992) 33. T.M. Sridhar, U. Kamachi Mudali, M. Subbaiyan, Corros. Sci. (submitted). J. Gluszek, J. Masalski, Brit. Corros. J. 27 (1992) 135. J.W. Oldfield, Int. Mate. Rev. 32 (1987) 153. P. Ducheyne, S. Radin, M. Heughebaert, J.C. Heughebaert, Biomaterials 11 (1990) 244. K.H.W. Seah, R. Thampuran, S.H. Teoh, Corros. Sci. 40 (1998) 547. T. Raghu, S.N. Malhotra, P. Ramakrishnan, Corrosion 45 (1989) 698. J.F. McIntyre, J.F. Leidheiser, Ind. Eng. Chem. Prod. Res. Dev. (1985). F. Deflorian, L. Fedrizzi, S. Rossi, P.L. Bonora, Electrochim. Acta 44 (1999) 4243. H.G. Pfaff, G. Willmann, R. Pothig, Properties of HA-coatings, in: P. Ducheyne, D. Christiansen (Eds.), Bioceramics, vol. 6, Butterworth-Heinemann, London, 1993, pp. 421–424. K. de Groot, J.G.C. Wolke, J.A. Jansen, Proc. Instr. Mech. Eng. 212 (1988) 137.